1. Field of the Invention
The invention relates to an electric power steering system that includes a cylindrical member that rotates coaxially with a rack shaft.
2. Discussion of Background
In an electric power steering system that includes a cylindrical member that rotates coaxially with a rack shaft, the rack shaft makes a reciprocating motion in its axial direction in response to a driver's operation of a steering wheel. As a result, the steered angle of steered wheels of a vehicle, that is, the travelling direction of the vehicle, is changed.
In some electric power steering systems, assist force is applied to a rack shaft from a motor. In such electric power steering systems, a ball screw nut through which the rack shaft is passed and which has a thread groove in its inner periphery is arranged, and the ball screw nut is rotated by the motor. A plurality of balls is arranged in a rolling passage that is formed between the thread groove of the ball screw nut, which serves as a cylindrical member, and a thread groove formed in the outer periphery of the rack shaft. A rotational motion that is generated by driving the motor is converted into an axial linear motion of the rack shaft via the ball screw nut and the balls.
In an electric power steering system, a bearing is arranged between a housing and a ball screw nut, and the ball screw nut is supported by the housing. WO 20081017541 A1 describes a configuration in which an elastic support mechanism is arranged. The elastic support mechanism elastically supports a bearing such that the bearing is movable (slightly movable) in the axial direction of a ball screw nut. Tooth noise is reduced by suppressing axial vibrations of the ball screw nut, and the steering feel at the start of turning a steering wheel is improved.
Specifically, as shown in FIG. 15, in each elastic support mechanism 90, an annular rubber member 91 that serves as an elastic member and an annular stopper 92 that serves as a supporting member are arranged in pair, and elastically support a bearing 93. The annular rubber member 91 is held between an end face 94a of an outer ring 94 of the bearing 93 and an inner end face 95a of a housing 95 with the annular stopper 92 interposed between the annular rubber member 91 and the outer ring 94, and elastically supports the outer ring 94 of the bearing 93 with respect to the housing 95.
The annular stopper 92 has an L-shape in section. The annular stopper 92 is held between the end face 94a of the outer ring 94 of the bearing 93 and an end face 91a of the annular rubber member 91, and is formed so as to be stretched between an inner periphery 95b of the housing 95 and an outer periphery 91b of the annular rubber member 91. When the annular rubber member 91 is not compressed with a predetermined load in the axial direction, the annular stopper 92 does not contact the inner end face 95a of the housing 95, and a clearance is formed between the annular stopper 92 and the inner end face 95a. On the other hand, when the annular rubber member 91 is compressed with the predetermined load in the axial direction, the annular stopper 92 contacts the inner end face 95a of the housing 95. Therefore, it is possible to prevent excess compression of the annular rubber member 91.
In such an electric power steering system, the annular stopper 92 is arranged on the outer periphery 91b of the annular rubber member 91. However, there is a possibility that the annular rubber member 91 may slip radially inward due to, for example, compression of the annular rubber member 91, and, as a result, the annular rubber member 91 may fail to deform in response to its compression and the driver's steering feel may deteriorate.